Method and system for measuring receiver mixer IQ mismatch

ABSTRACT

A system for correcting a mismatch between an in-phase path and a quadrature path in a receiver, the system including one or more processors operable to correlate a received signal with a complex training sequence having periodic, uncorrelated real and imaginary components, thereby generating a complex correlation. The one or more processors estimate a gain mismatch and a phase mismatch based on the complex correlation over a unit period. The one or more processors control a first receive filter in the in-phase path via a first digital-to-analog controller, and control a second receive filter in the quadrature path via a second digital-to-analog controller. The cutoff frequency and bandwidth of the first receive filter is controlled independently of the second receive filter to enable the output of the first receive filter and the output of the second receive filter to be tightly matched.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.12/568,927 filed Sep. 29, 2009, which is a continuation of U.S.application Ser. No. 11/668,943 filed Jan. 30, 2007 (now U.S. Pat. No.7,596,363), which in turn makes reference to, claims priority to andclaims the benefit of U.S. application Ser. No. 10/774,116 filed Feb. 6,2004 (now U.S. Pat. No. 7,187,916), which in turn makes reference to,claims priority to and claims the benefit of: U.S. Provisional PatentApplication Ser. No. 60/445,525 filed on Feb. 7, 2003.

FIELD OF THE INVENTION

The present invention relates to transceivers, and more particularly toestimating the receiver mixer IQ mismatch and digitally compensating themismatch.

BACKGROUND OF THE INVENTION

One increasingly popular application for wireless systems are wirelesslocal area networks (WLANs) of computer systems. Prominent in the fieldof home and business, wireless computer networks include the wirelessstandards known as 802.11. The first standard to be available incommercial products was 802.11b. However, increasing demand for highercapacity in the growing wireless LAN market has led to the introductionof a new generation of WLAN standards using more spectrally efficientmodulation techniques, including the IEEE 802.11a standard. The 802.11astandard operates in the 5 GHz unlicensed national informationinfrastructure (UNII) band (5.15-5.35 GHz, 5.725-5.825 GHz) and is basedon orthogonal frequency division multiplexing (OFDM). It supports datarates from 6 Mb/s to 54 Mb/s compared to 1 Mb/s to 11 Mb/s offered by802.11b. The 802.11a operation around 5 GHz offers the additionaladvantage of less interference compared to the 2.4 GHz ISM band, wherein addition to 802.11b, other band users include microwave ovens,cordless phones, Bluetooth systems, and vintage 802.11 systems.

The advantages of 802.11a come at a cost, however, as OFDM-based systemspose significant implementation challenges requiring low in-band phasenoise, high linearity, accurate quadrature matching, closely matchedfrequency response between the I and Q signal paths, and a large dynamicrange. “I” and “Q” are terms referring to “in-phase” and “quadrature,”respectively. Ideally, the difference between I and Q signal paths in atransceiver is 90 degrees and the gain is 0 dB. I/Q mismatch refers tothe difference in phase and gain between these paths. For example, inorder to meet the transmitter error vector magnitude (EVM) specificationfor the 54 Mb/s mode with a 3 dB implementation margin, systemsimulation shows that an I/Q mismatch of 1.5o/0.2 dB, an integratedphase noise error of 1o rms and operation at 8 dB backoff from thetransmitter 1 dB compression point are required.

In addition to tight performance constraints, pricing pressures requirethat wireless systems be low-cost and highly integrated implementations.To address these needs, the continuous trend towards low-costintegration of wireless systems has driven the introduction ofinnovative single-chip architectures in CMOS technologies as inexpensivealternatives to the traditional superheterodyne implementationsoperating at frequencies up to 5 GHz. Many of these single chiparchitectures are homodyne or direct conversion architectures, whichhave much fewer components than superheterodyne implementations. Forexample, in superheterodyne systems, the intermediate frequency (IF)must be high, so that the image is located far from the wanted signal inthe frequency spectrum. However, since the IF frequency is high, thefiltering of the desired channel (at IF) must also be done at the highfrequency. It is difficult or impossible to implement such a filter asan integrated system, so external components are unavoidable. Directconversion systems do not need such external components.

However, such highly-integrated direct-conversion single-chiparchitectures suffer from well-known shortcomings that may limit theirapplicability. These are problems that are also shared by otherintegrated architectures such as low-IF or wideband-IF, but which can befurther aggravated by using CMOS technology.

For example, on the receiver side, the most common problem is thepresence of DC offsets, both static and time-varying. In the 802.11astandard, even though a down-converted I/Q signal occupies bandwidthfrom 150 kHz to 8.3 MHz, the maximum 40 ppm frequency mismatch allowedbetween transmitter and receiver may shift the signal around DC, thusprohibiting AC coupling without using complex analog frequencycorrection techniques.

Static DC offset is the result of component mismatches in the signalpath and local oscillator (LO) leakage at the inputs of the mixer andthe low-noise amplifier (LNA) due to finite on-chip isolation. Theleakage signal after mixing with the LO produces a DC component at thebaseband input, which depends on the frequency and power of the LOsignal. Since static DC offset may be large enough to saturate thebaseband receive chain, it needs to be cancelled in the analog domain.

Time-varying DC offsets, in direct conversion receivers, can be theresult of self-mixing due to leakage of single-tone (CW) or frequencymodulated (FM) interference to the LO port. Similarly, second orderdistortion applied to CW or FM interference results in DC offset, whichvaries with the frequency and the power level of the received signal.Since strong interference is not usually present in the 802.11aoperating bands, the dominant mechanism causing time-varying DC offsetsis self-mixing of the LO signal leaking to the antenna and reflectedback from the environment. At the 5 GHz carrier frequency, due to highattenuation and absorbency of reflected signals, such time-varying DCoffsets are small compared to the static DC offsets and the overalldynamic range of the receiver (e.g., in the order of 10-50 mV for a2Vp-p signal), thus it is well known that the time-varying offsets canbe tracked and removed by digital signal processing (DSP) afteranalog-to-digital conversion.

Direct down/up-conversion from/to 5 GHz requires quadrature LOgeneration at the RF carrier frequency which may result in large I/Qmismatches (including gain and phase mismatches). Other significantproblems include sensitivity to flicker noise and pulling of thevoltage-controlled oscillator (VCO) by the external or on-chip poweramplifier (PA). In addition to these architecture-relatednon-idealities, higher order QAM-OFDM modulation requires tightlymatched baseband I/Q filters on both transmit and receive side to avoiddegradation of the overall EVM.

For a spectrum efficient modulation such as QAM64, precise IQ pathmatching is required for both phase and gain A phase mismatch of a fewdegrees or a gain mismatch of 1 db would have a significant impact onthe overall performance of the communication system. Achieving such ahigh matching by analog design and manufacturing technology is not easy,because of limited manufacturing accuracy.

Accordingly, what is needed is an efficient way to digitally measure thereceive path IQ mismatch, estimate the mismatch as a calibration processand then to digitally compensate it. The present invention addressessuch needs.

SUMMARY OF INVENTION

The present invention provides a method and system for measuringreceiver mixer IQ mismatch in a transceiver. The measuring includesproviding a training signal for a receiver mixer, the training signalhaving periodic, uncorrelated I and Q signals. A phase mismatch in thereceiver mixer is determined from IQ correlation over a unit period. Again mismatch in the receiver mixer is determined from a power estimateof both I and Q signal for the unit period.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an overall system architecturefor a direct conversion transceiver and calibration processor of thepresent invention;

FIG. 2 is a block diagram illustrating the transceiver of the presentinvention;

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to estimating the receiver mixer IQmismatch and digitally compensating the mismatch. The followingdescription is presented to enable one of ordinary skill in the art tomake and use the invention and is provided in the context of a patentapplication and its requirements. Various modifications to the preferredembodiment and the generic principles and features described herein willbe readily apparent to those skilled in the art. Thus, the presentinvention is not intended to be limited to the embodiments shown but isto be accorded the widest scope consistent with the principles andfeatures described herein.

Embodiments and examples of the present invention are described below.While particular applications and methods are explained, it should beunderstood that the present invention can be used in a wide variety ofother applications and with other techniques within the scope of thepresent invention.

FIG. 1 is a block diagram of an overall system architecture 10 for thepresent invention. System 10 includes one or more antennas 12, atransfer switch 14, a radio frequency (RF) transceiver 16, a companionbaseband processor 18, and calibration methods 20.

Antennas 12 are used to receive a transmitted signal from the air, andto transmit a signal to this air channel. For example, in a wireless LANenvironment, a different computer may be transmitting data via wirelesstransmission, which is then received by one or more antennas 12. Or, thecomputer to which the antennas 12 are coupled can transmit a signal viaantennas 12, which is received by a different antenna, computer ordevice. Other types of wireless devices can also use one or moreantennas 12 which are also suitable for use with the present invention.A transfer switch 14 can be coupled to the antennas 12 and is used toswitch between an input port 22 and an output port 24. The transferswitch 14 can be a double-pole double-throw (DPDT) switch, or acombination of switches or switches and diodes performing the samefunctionality. Typically, the received signal is filtered by one or moreradio frequency (RF) filters (not shown) before being provided at thereceiver input 12.

The input 22 and output 24 are connected to RF transceiver 16 of thepresent invention, which includes a receiver component 30, a transmittercomponent 32, and a frequency synthesizer 34. The receiver component 30receives the signals from the antennas 12 and processes the signals toallow the desired data to be retrieved from the signals. Processingincludes frequency translation to a low intermediate frequency (IF) orbaseband, and filtering of unwanted interference from the useful signal.The transmitter component 32 converts a signal formulated by thebaseband processor 18 (or other processor) to a form in which it can betransmitted by the antennas 12. Frequency synthesizer 34 generates thereference frequency (LO) signals needed for modulation and demodulationin the transceiver 16. In one described embodiment, transceiver 16 isprovided on a single integrated circuit chip, an integrated solutionallowing it to be cheaply manufactured. RF transceiver 16 is describedbelow in greater detail with respect to FIG. 2.

In one application for the present invention, transceiver 16 can beimplemented as a direct conversion radio transceiver. This embodimentcan implement the radio part of the PHY (physical) layer for an 802.11aWLAN system, for example, or other wireless LAN or communication system.For example, CMOS can be used for a single-chip implementation. Many ofthe problems that typically apply to integrated CMOS or other similarintegrated technology in direct conversion high frequency transceiverapplications are minimized in the present invention through the use ofcalibration techniques and overall system design.

Baseband processor 18 is coupled to RF transceiver 16. Processor 18 canbe any type of processor, including a microprocessor,application-specific integrated circuit (ASIC), or other type ofcontroller. Processor 18 can be a general purpose processor thatperforms other tasks and functions for the device in which the system 20is included in addition to the functions needed for the presentinvention. For example, processor 18 can be a controller or processor ina wireless network card, or a general microprocessor in a computer thatincludes a wireless network card using the present invention.Alternatively, processor 18 can be specifically provided for the presentinvention, and in some embodiments can be included with transceiver 16,e.g. on the same integrated circuit chip. One important feature of someembodiments of the present invention is that the functionality of theprocessor 18 needed for the present invention can be implemented usingfree resources of an existing processor, such as a host processor, andhas little impact on the computational resources of such a processor.

An important function of the processor 18 for the present invention isthe compensation of non-idealities of the transceiver 16 using computingresources of the processor 18. The baseband processor 18 receives adown-converted received signal from the receiver component 30 of thetransceiver 16, where the signal has been separated into its in-phase(I) and quadrature (Q) forms. The received signal is provided toanalog-to-digital converters (ADCs) 38 a and 38 b, where ADC 38 areceives the I signal and ADC 38 b receives the Q signal.

The digital outputs of the ADCs 38 a and 38 b are provided to a digitalcorrection block 40, which performs real-time I/Q mismatch correction(post-distortion) by using the coefficients computed.

The digital correction block 40 provides the I and Q data to a digitalsignal processor (DSP) 42, which performs the rest of the functionalityof the PHY and MAC (media access control) communication layers (e.g.,communicating with other processors and/or functions of a host computeror device. This functionality can be implemented entirely within the DSPprocessor 42, be partitioned between a processor (such as an ASIC) and ageneral processor, or reside totally in a general (e.g., host)processor. The DSP 42 can be a processor on the processor chip 18, orsome other processor used to implement the calibration methods describedherein. In receive mode, DSP 42 receives corrected digital data from thedigital correction block 40, representing signals received by thetransceiver 16, for digital demodulation for recovery of the originalbits of information.

In transmit mode, when the DSP 42 has data to be transmitted, the datais corrected in correction block 44, which performs real-time I/Qmismatch correction (pre-distortion) by using the coefficients(calibration parameters) computed during an initial calibration cycle.

Correction block 44 sends the I data to digital-to-analog controller(DAC) 46 a and sends the Q data to DAC 46 b. DACs 46 a-b convert thedigital data into I and Q analog signals which are provided to thetransmitter 32 of transceiver 16. In one embodiment, DACs 46 a-b can bedual 10-bit 40 MHz DACs, but may be any suitable type in otherembodiments.

DSP 42 also provides programming signals 50 for the RF transceiver 16,which can include (but are not limited to) real time automatic gaincontrol (AGC) signals, and signals for programming the frequencysynthesizer 34, programming the correction DC offset for the DC offsetcorrection DACs 120 and 122 of the receiver (see FIG. 2), programmingthe cutoff for the analog filters 132, 134, 210, 212 (see FIG. 2), andprogramming the chip into different states or calibration modes. In thedescribed embodiment, a digital interface 48 of the transceiver 16receives these signals which can include, for example, a 7-bit AGCcontrol line and a 3-wire interface for all other programming as well asreal-time signals for receiver and transmitter switching. Theprogramming signals 50 allow the processor 18 to control loop-back pathson the transceiver 16 so that the processor can send signals (stimuli)to the transceiver and to receive responsive signals from thetransceiver based on the signals, i.e., as if the transceiver hasreceived the signals wirelessly. This enables the measurement of analogimperfections of the transceiver 16 and allows the processor 18 to makeadjustments, also using the programming signals 50, to components in thetransceiver to minimize or compensate for problems, such as I/Qmismatches, baseband filter cutoff frequency mismatch, DC offset, etc.The programming signals 50 can control the calibration paths needed forDC offset calibration, filter tuning, I/Q mismatch calibration, and LOleakage calibration, and can also be used for tuning of the frequencysynthesizer 34. For example, processor 18 preferably has the capabilityof real-time digital pre-distortion and post-distortion in blocks 44 and40, respectively, which is needed for compensation of transmitter andreceiver I/Q mismatch and transmitter local oscillator (LO) leakage.

Baseband processor 18 can be implemented in a variety of differentembodiments. The functions of processor 18 shown in FIG. 1 are common inmany types of microprocessors and other types of processors, or canexist in specialized processors (e.g., ASICs), such as for wireless LANapplications in such an embodiment.

Methods 20 calibrate the transceiver 16 by measuring characteristics oftransceiver components based on received signals by and computing thenecessary correction parameters to correct any analog imperfections inthe circuits. These methods can be implemented by the processor 18 insome embodiments (such as by DSP 42), for example as programinstructions or code (software or firmware) stored in memory or otherstorage device (magnetic disk, CD-ROM, etc.), or by hardware (logicgates, etc.), or by a combination of hardware and software. In otherembodiments, the methods can be implemented and performed by a generalpurpose processor, such as the host processor of a computer thatincludes a network card or component holding the system 20, or by adifferent processor in communication with the processor 18 ortransceiver 16.

The present invention thus enables the correction for nonidealities inthe analog front-end of a transceiver by using overall system resourcesand achieves performance adequate to support high bit rate modulationschemes. In some embodiments, aspects of the present invention can alsobe used in non-direct-conversion transceivers, as appropriate.

FIG. 2 is a schematic diagram illustrating one embodiment of thetransceiver 16 of the present invention. As described above, transceiver16 includes a receiver 30, a transmitter 32, and a frequency synthesizer34. Both transmitter and receiver components preferably use directconversion and employ fully differential signal paths to minimizecrosstalk and externally induced noise.

Receiver

The receiver 30 includes a receiver input 112, which provides a signalreceived by the transceiver 16 from the antenna 12. The signal ispreferably in differential form to reduce second order distortion andminimize substrate and supply coupling, such that positive (P) andnegative (N) terminals are used; however, signals in other forms can beused with the present invention in alternate embodiments.

The received signal is sent from the input 112 to a low noise amplifier(LNA) 114, which amplifies the received signal. The LNA has sufficientgain to provide a signal having a high enough amplitude for use in thereceiver while introducing little noise to the signal.

The amplified signal from LNA 114 is provided to a quadraturedemodulator 116, which downconverts the high frequency signal directlyinto the baseband frequency. The quadrature demodulator 116 includes twomixers in the described embodiment. The quadrature demodulator 116receives a reference (LO) signal at a desired frequency from a frequencysynthesizer 34 The demodulator 116 separates the received passbandsignal into two baseband differential signals, one In-phase (I) and oneQuadrature (Q).

The quadrature demodulator 116 can introduce DC offset to the signal.Large DC offset can corrupt a weak signal or saturate the basebandcircuits, programmable gain amplifiers (PGAs) 124 and 126, filters 128and 130, etc. The overall receive chain path DC offset is calculated andis real-time corrected (adaptively cancelled) at the output of themixers of the quadrature demodulator 116 by two independent 8-bitcurrent steering digital-to-analog converters (DACs) 120 and 122. DAC120 corrects the DC offset of the I path of the demodulator 116. and DAC122 corrects the DC offset on the Q path of the demodulator 116, byadding a small DC voltage (relative to the DC voltage level) to thesignal at the output of the demodulator 116 and input to the PGA 124 or126.

The baseband path of the receiver includes components of amplifiers,filters, and output buffers on the I and Q paths. The baseband path caninclude different components in other embodiments; a direct-conversionbaseband path, in general, performs filtering and variable gain.

Digitally programmable gain amplifiers (PGAs) 124 and 126 receive theoutputs of the demodulator 116 on the I and Q paths, respectively. PGA124 can employ, for example, a low-noise, high dynamic rangesingle-stage amplifier with a resistive attenuator at its input, but canbe other types of programmable amplifiers in alternate embodiments. Thegain of the PGA 124 and 126 is programmable by a digital word providedby the baseband processor 18, e.g., via the AGC programming bus 50.Other embodiments can also be used, such as variable gain amplifiers(VGAs) controlled by an analog voltage.

Low pass filters 128 and 130 receive the output of the corresponding PGA124 or 126 on the I and Q paths, respectively, and perform basebandchannel selection. In one embodiment, a fourth order Chebyschev filtercan be used for each filter 128 and 130; however, any filter can be usedwhich can be tuned by a voltage or by a digital word. The response ofeach filter 128 and 130 can be tuned to a desirable cutoff frequency andbandwidth by the DC voltage Vc generated by DACs 132 and 134,respectively. The DACs are controlled by signals from the processor 18to enable the receive filters to be tightly matched in their responseswith each other.

After the low pass filters 128 and 130, two secondary PGAs 136 and 138are employed, where PGA 136 is used on the I path, and PGA 138 is usedon the Q path. In the described embodiment, each PGA 136 and 138 is anoperational amplifier-based feedback gain stage; other types ofamplifiers can be used in other embodiments. To achieve optimumnoise/linearity performance, the two PGAs 124 and 136 (or 126 and 138)are located before and after the channel-select filter 128 (or 130). Inone example of the described embodiment, the composite gain of theamplifiers along a path (I or Q) can vary from 2 dB to 53 dB,programmable in 3 dB steps by an external 6-bit word provided by theprocessor 18. In general, PGAs 124/126 and 136/138 perform gain control.In other embodiments, the PGAs 124/126 and/or 136/138 can be merged withthe filters 128 and 130 rather than being provided as separatecomponents. Or, only one PGA (or other type of variable gain or fixedamplifier) can be after the filter or before the filter; or, one fixedamplifier is provided before and a variable gain amplifier after (orvice versa), etc.

An output buffer 140 and 142 is employed at each of the I and Q paths,respectively, after the PGAs 136 and 138. The outputs of the buffers 140and 142 are provided at receiver outputs 144 and 146 of the transceiver16, where each path provides a differential signal having positive andnegative components. The signals from these receiver outputs areprovided to processor 18 as explained above with respect to FIG. 1.

Transmitter

The transmitter 32 of transceiver 16 is a direct conversion,up-conversion transmit path and includes transmitter inputs 200 and 202.The inputs 200 and 202 receive a signal from the baseband processor 18which has formulated the analog signal that is to be upconverted andtransmitted by the transceiver 16. For example, in the describedembodiment, the transmit path input analog signal is provided by DACs 46a and 46 b located at the processor 18, as described above with respectto FIG. 1. The signal is preferably in the form of I and Q signals,where input 200 receives the I signal, and input 202 receives the Qsignal. Furthermore, each I and Q signal path is preferably provided indifferential form, such that positive (P) and negative (N) terminals areused by each.

The I and Q signals to be transmitted are sent from the inputs 200 and202 to input buffers 204 and 206, respectively. From the outputs of thebuffers 204 and 206, the I and Q signals are sent to transmit basebandfilters 208 and 210, respectively. These filters are low pass filters,e.g., they can be the same or similar to the filters 128 and 130 used inthe receiver of the transceiver 16 as described above. Similarly to thelow pass filters of the receiver, the responses of the transmit filters208 and 210 can be preferably tuned to the desirable cutoff frequencyand bandwidth by the DC voltage Vc generated by 8-bit DACs 212 and 214,respectively. In other embodiments, variable gain stages can be includedbefore, after, or merged with the filter.

The outputs of the filters 208 and 210 provide the filtered I and Qsignals to a programmable gain modulator (PGM) 216. In other embodimentsthe modulator 216 can be fixed gain. The PGM 216 up-converts the I and Qpathways to a single transmit path at the desired RF carrier frequency,i.e., the mixer modulates the baseband signals directly into the highfrequency signal. Similar to the quadrature demodulator 116, the PGM 216receives a reference (LO) signal at a desired frequency from frequencysynthesizer 34. PGM 216 utilizes two mixers.

The output of the I and Q mixers in the PGM 216 is summed in order todrive the RF output amplifier. Gain programmability of the PGM 216 isachieved by adjusting the input transconductance stage using aswitchable resistive ladder at the input of the mixers. In one exampleembodiment, the modulator 216 provides 27 dB of gain control in 3 dBsteps.

The output of the PGM 216 is preferably a fully differential signalpath, but in alternate embodiments can take other forms. Thedifferential output of the PGM 216 is amplified by an RF output driveramplifier 218, also preferably differential. In the describedembodiment, the driver amplifier 218 can be a single stage cascodeddifferential pair, inductively degenerated to improve linearity. Inother embodiments, amplifier 218 can have variable programmable gain.The RF amplifier amplifies the signal from the modulator 216 to deliverthe signal to the antenna 12. The signal can be then sent to an externalpower amplifier, can be first filtered first and then delivered to anexternal power amplifier, or can be filtered and delivered to theantenna, e.g., via the switch 14.

The output of the driver amplifier 218 is converted to a single-endedsignal (in most embodiments) via a balanced-to-unbalanced converter(BALUN) or transformer 222, the output of which is then applied to theinput of an external power amplifier (PA) 224. In the describedembodiment, the amplifier 224 and transformer 222 are shown located offof the chip of the transceiver 16 and receiving the signal via theoutput 220 of the chip; however, the amplifier 224 and/or transformer222 be located on the chip in other embodiments.

Envelope detector 228 has an input connected to the driver amplifier 218for receiving the output RF transmit signal and detects an envelope ofthe signal, e.g. amplitude demodulates the signal. The detector 228 hasan output connected to the I path of the receiver 30 after PGA 136 viaswitch SW7. When SW7 is enabled (“closed”) then the envelope detector228 is connected to the I channel and provides the envelope of thetransmitted signal to the analog I output of the receiver and thus toprocessor 18. This signal path is active in calibration mode, when thetransmit I/Q mismatch and LO leakage are measured. When SW7 is inactive(“open”), then the envelope detector 228 is not in use. Envelopedetector 228 can be included on the chip of the transceiver 16, or bemade external to the transceiver chip. Also, in other embodiments,detector 228 can be located after the PGM 216 or after the PA 224, orcan be part of the PA 224.

A number of “loop-back” switches are provided in the transceiver 16. Theswitches allow desired feedback or loopback paths of the transceiver 16to be selected to be active so that calibration can occur.

Switches SW1, SW2, and SW5 enable measurement of frequency response ofthe receiver baseband filters. Switches SW3 and SW4 similarly enablemeasurement of the frequency response of the transmitter basebandfilters 208 and 210, which can then be digitally calibrated to thedesired cutoff by DACs 212 and 214.

Switches SW6 and SW7 are used for transmitter and receiver I/Q mismatchmeasurement. When switch SW7 is closed and the other switches open, theRF output of the transmitter is output from driver 218, through theenvelope detector 218 to the I-path at the receiver output, through thereceiver buffer 140, and back to processor 18. This allows a calibrationwaveform sent to the transmitter to be amplitude demodulated by theenvelope detector, bypass the receiver, and fed back to the processor18, where the envelope of the RF signal is digitized by the ADC 38 a ofthe processor 18. The detected envelope is used to jointly measuretransmitter I/Q mismatch and LO leakage. Alternatively, the calibrationpath of SW7 can go to any other dedicated pin or connection that can bereceived and digitized by the processor, or to any other point of thebaseband receive chain, I or Q, as long as there is a way for thebaseband processor 18 to read the information.

Switch SW6 enables a feedback loop between the transmitter and thereceiver. When switch SW6 is closed (and all others open), a path isformed from the output of PGM 216 on the transmitter to the input of thequadrature demodulator 116 of the receiver. Thus a calibration waveformcan be sent from the transmitter 32 to the front of the receiver 30, tobe received by processor 18 through the receiver. The received waveformcan be used in measuring receiver I/Q mismatch, as described in moredetail hereinbelow.

Digital interface 48 provides the interface to the transceiver 16 toallow the control/programming signals from the processor 18 to power upor down different sections of the transceiver, e.g., to make sure that,in various calibration modes, different sections of the transceiver arepowered up and down (the components/sections powered up are generallythose where the signal passes through or necessary for signal passage).Interface 48 also allows the signals from processor 18 to operatecontrol circuitry on the transceiver, such as the DACs 120, 122, 132,134, 212 and 214, and the control switches (described above) to createthe necessary loopback paths. The digital interface 48 can send commandsthat are decoded in the transceiver (by logic, etc.) which sends theappropriate control signals or digital words to the switches, DACs,etc., and to power up and down different sections.

Receiver IQ Mismatch Measurement In accordance with a preferredembodiment of the present invention, receiver I/Q mismatch is measuredby the processor 18. Switch SW6 is closed while other switches remainopen to make the receiver mismatch measurement loopback connectionactive. The transmitter circuit generates RF training signals just likeduring an ordinary data transmission. The RF signals are then fed to thereceiver mixer. They travel through regular receiver signal path untilthey are measured by the I and Q ADCs 38 a and 38 b. To distinctlyestimate the receiver mixer IQ mismatch, the transmitter mixer mismatchand other IQ mismatch has to be accurately matched. By way of example,an envelope of a transmitted calibration signal is measured for leakageof the LO signal of the frequency synthesizer 34 and for transmitter I/Qmismatch. To do this in the described embodiment, switch SW7 is closedwhile the other switches remain open to make the appropriate loopbackconnection active, and a calibration waveform is sent to the transmitter32 so that the envelope of the waveform, provided by envelope detector228, is digitized by ADC 38 a of the processor 18. Methods for envelopedetection of a simple waveform, measurement of LO leakage and transmitI/Q mismatch from the envelope, and determination of calibrationparameters for the leakage and transmit I/Q mismatch are well known,e.g., in J. K. Cavers, “New methods for adaptation of quadraturemodulators and demodulators in amplifier linearization circuits,” IEEETransactions on Veh. Technology, vol. 46, no. 3, pp. 707-716, August1997.

In a preferred embodiment, the training signal from the transmitter thatis used for the receiver mixer IQ mismatch has the followingcharacteristics: I and Q signals are periodical and uncorrelated. Thesefeatures are necessary for a simple and accurate estimation, andcombined with the estimation algorithm, the estimation becomesinsensitive from phase rotation between the transmitter and receivermixers, which could introduce estimation error or require a morecomplicated estimation calculation. Further, with these features for theestimation, the phase mismatch can be simply calculated from IQcorrelation taken over the unit period, while the gain mismatch can besimply calculated from the power estimate of both of I and Q for theunit period.

In a further embodiment, the estimation could operate in an iterativefashion. One of the advantage of the iterative estimation is that itlifts the small mismatch requirements the estimation algorithm has. Asit operates as an iterative way naturally the residual mismatch becomessmaller, and it very quickly satisfies the small mismatch requirements.Hence, it gives an accurate estimate. This iterative estimation can bedone in two ways:

1) By using a pre-compensated signal as a measurement signal, it canestimate the residual of the receiver mixer IQ mismatch.

2) By performing post-correction using the latest available correctionparameter and applying the algorithm on the post-corrected signals tofurther estimate the residual mismatch. This mismatch value is againiteratively used to further perform the post-correction and so on.Mismatch compensation can be performed by a matrix multiplication. Oneof the cross term can be set to zero by further applying a certainrotation, which has no impact for demodulation performance.

In measuring the receiver mixer IQ mismatch, a system description for asmall mismatch case can be given as:

$\begin{pmatrix}I^{\prime} \\Q^{\prime}\end{pmatrix} = {\begin{pmatrix}{1 + g} & f \\f & {1 - g}\end{pmatrix}\begin{pmatrix}{\cos\;\theta} & {{- \sin}\;\theta} \\{\sin\;\theta} & {\cos\;\theta}\end{pmatrix}\begin{pmatrix}I \\Q\end{pmatrix}}$

This computes to:I′={f·Q+(1+g)·I} cos θ+{f·I+(1+g)·Q} sin θQ′={f·I+(1−g)·Q} cos θ+{−f·I+(1−g)·I} sin θIf a correlation is taken between I′ and Q′ for the unit period thefollowing can be shown:

${\sum\limits_{period}{I^{\prime} \cdot Q^{\prime}}} = {{f \cdot \left\{ {{\sum\limits_{period}I^{2}} + {\sum\limits_{period}Q^{2}}} \right\}} = {{f \cdot {Power}}\mspace{14mu}{where}}}$${Power} = {\sum\limits_{period}\left\{ {I^{2} + Q^{2}} \right\}}$

For the above, the second order terms in terms of T and ‘g’ are ignored.

Thus, the correlation does not depend on ‘θ’ but only on ‘f’ and power,and the IQ phase mismatch parameter ‘f’ can be calculated as follows.

$f = \frac{\sum\limits_{period}{I^{\prime} \cdot Q^{\prime}}}{Power}$

Similarly a power for I and Q over the unit period may be calculated as:

${\sum\limits_{period}I^{\prime 2}} = {\left\{ {1 + {2 \cdot g}} \right\} \cdot {Power}}$${\sum\limits_{period}Q^{\prime 2}} = {\left\{ {1 - {2 \cdot g}} \right\} \cdot {Power}}$

Again, the I′ and Q′ power does not depend on ‘θ’ but only on ‘g’ andpower.

Thus, the IQ gain mismatch parameter ‘g’ can be calculated as follows.

$g = \frac{{\sum\limits_{period}I^{\prime 2}} - {\sum\limits_{period}Q^{\prime 2}}}{4 \cdot {Power}}$

Digital pre-distortion or post distortion can be done as follows usingthe calculated parameters when ‘f’ and ‘g’ are small.

$\begin{pmatrix}I^{\prime} \\Q^{\prime}\end{pmatrix} = {\begin{pmatrix}{1 + g} & f \\f & {1 - g}\end{pmatrix}\begin{pmatrix}I \\Q\end{pmatrix}}$

In general, especially when the mismatch is large, the digitalpre-distortion or post-distortion becomes a general matrixmultiplication:

$\begin{pmatrix}I^{\prime} \\Q^{\prime}\end{pmatrix} = {\begin{pmatrix}a & b \\c & d\end{pmatrix}\begin{pmatrix}I \\Q\end{pmatrix}}$

By introducing an iterative correction and estimation procedure, thecorrection parameters

‘a’, ‘b’, ‘c’ and ‘d’ can be accurately acquired. The procedure of theestimation is:

-   1) Calculate the post-distorted I′ and Q′ signals using the latest    available, digital compensation parameters.-   2) Apply the algorithm over these. I′ and Q′-   3) Estimate the latest mismatch parameters-   4) Calculate the updated compensation parameters

The following provides a more particular example of the estimationimplementation. As far as I and Q signals are uncorrelated and periodic,with a same power any signals would work. One example for such signalsis the shorter preamble sequence used in 802.11a standard. The followingis the code for the generation of the signal, sig(1:16):

-   S53=[0,0,1+j,0,0,0,−1−j,0,0,0,1+j,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1+j,0,0,0,    . . .    0,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0];-   S53=S53*sqrt(13/6.);-   sx(1:27)=S53(27:53);-   sx(39:64)=S53(1:26);-   yx=ifft(sx);-   sig=yx(1:16);

From the measured I and Q signals, rxI(1:LEN), rxQ(1:LEN), the phasemismatch can be calculated as follows:pow=sum(abs(complex(rxI,rxQ)).^2);ccorr=sum(rxI.*rxQ);f=−2*ccorr/pow;

From the measured I and Q signals, rxI(1:LEN), rxQ(1:LEN), the gainmismatch can be calculated as follows:powI=sum(rxI.^2);powQ=sum(rxQ. ^2);g=(powQ−powI)/2/(powI+powQ);

The codes for the digital pre-distortion or post-distortion would be:I=(1+g)*I+PQ;Q=f*I+(1−g)*Q;

The following provides an example of entire algorithm implementationusing an iterative algorithm:

alph=0.6; beta=0.6; a=1; b=0; c=0; d=1; for I=1:Iter rxIcorr=a*rxI+b*rxQ;  rxQcorr=c*rxI+d*rxQ; pow=sum(abs(complex(rxIcorr,rxQcorr)).{circumflex over ( )}2);  ccorr =sum(rxIcorr .* rxQcorr);  f = −2*alph*ccorr/pow; powI=sum(rxIcorr.{circumflex over ( )}2);  powQ=sum(rxQcorr.{circumflexover ( )}2);  g=beta*(powQ−powI)/2/(powI+powQ);  a=a*(1+g)+c*f; b=b*(1+g)+d*f;  c=a*f+c*(1−g);  d=b*f+d*(1−g); endHere, ‘alph’ and ‘beta’ is a parameter introduced for the stability. Thefinally obtained parameters ‘a’, ‘c’ and ‘d’ are the parameters to beused for the digital corrections.

Through the present invention, the training signal used for the receivermixer IQ mismatch provides I and Q signals that are periodical anduncorrelated. In this manner, a simple and accurate estimation isachieved, that remains substantially insensitive to phase rotationbetween the transmitter and receiver mixers. Further, with thesefeatures for the estimation, the phase mismatch can be simply calculatedfrom IQ correlation taken over the unit period, while the gain mismatchcan be simply calculated from the power estimate of both of I and Q forthe unit period. Additionally, the estimation is capable of operating inan iterative fashion.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

1. A system for correcting a mismatch between an in-phase path and aquadrature path in a receiver, the system comprising: one or moreprocessors operable to: correlate a received signal with a complextraining sequence having periodic, uncorrelated real and imaginarycomponents, thereby generating a complex correlation; estimate a gainmismatch and a phase mismatch based on the complex correlation over aunit period; control a first receive filter in the in-phase path via afirst digital-to-analog controller; and control a second receive filterin the quadrature path via a second digital-to-analog controller,wherein the cutoff frequency and bandwidth of the first receive filteris controlled independently of the second receive filter to enable theoutput of the first receive filter and the output of the second receivefilter to be tightly matched.
 2. The system of claim 1, wherein said oneor more processors provides said complex training sequence on a closedRF signal path between a transmitter and a mixer of said receiver. 3.The system of claim 1, wherein said one or more processors estimatessaid gain mismatch and said phase mismatch iteratively.
 4. The system ofclaim 3, wherein said one or more processors utilizes a pre-compensatedsignal as a measurement signal for said iterative estimation.
 5. Amethod for correcting a mismatch between an in-phase path and aquadrature path in a receiver, the method comprising: correlating areceived signal with a complex training sequence having periodic,uncorrelated real and imaginary components, thereby generating a complexcorrelation; estimating a gain mismatch and a phase mismatch based onthe complex correlation over a unit period; controlling a first receivefilter in the in-phase path via a first digital-to-analog controller;and controlling a second receive filter in the quadrature path via asecond digital-to-analog controller, wherein the cutoff frequency andbandwidth of the first receive filter is controlled independently of thesecond receive filter to enable the output of the first receive filterand the output of the second receive filter to be tightly matched. 6.The method of claim 5, comprising providing said complex trainingsequence on a closed RF signal path between a transmitter and a mixer ofsaid receiver.
 7. The method of claim 5, comprising estimating said gainmismatch and said phase mismatch iteratively.
 8. The method of claim 7,comprising utilizing a pre-compensated signal as a measurement signalfor said iterative estimation.
 9. A system for correcting a mismatchbetween an in-phase path and a quadrature path in a receiver, the systemcomprising: one or more processors operable to: correlate a receivedsignal with a complex training sequence having periodic, uncorrelatedreal and imaginary components, thereby generating a complex correlation;estimate a gain mismatch and a phase mismatch based on the complexcorrelation over a unit period; and apply at least one correctionparameter to the receiver, wherein said at least one correctionparameter is based on said gain mismatch estimate and said phasemismatch estimate.
 10. The system of claim 9, wherein said one or moreprocessors provides said complex training sequence on a closed RF signalpath between a transmitter and a mixer of said receiver.
 11. The systemof claim 9, wherein said one or more processors estimates said gainmismatch and said phase mismatch iteratively.
 12. The system of claim 9,wherein said one or more processors estimates said at least onecorrection parameter iteratively.
 13. The system of claim 11, whereinsaid one or more processors utilizes a pre-compensated signal as ameasurement signal for said iterative estimation.
 14. The system ofclaim 11, wherein said one or more processors performs post-correctionusing a latest available correction parameter, and estimates residualmismatch from post-correction signals.
 15. The system of claim 14,wherein said one or more processors performs mismatch compensation usingmatrix multiplication, during said post-correction.
 16. A method forcorrecting a mismatch between an in-phase path and a quadrature path ina receiver, the method comprising: correlating a received signal with acomplex training sequence having periodic, uncorrelated real andimaginary components, thereby generating a complex correlation;estimating a gain mismatch and a phase mismatch based on the complexcorrelation over a unit period; and applying at least one correctionparameter to the receiver, wherein said at least one correctionparameter is based on said gain mismatch estimate and said phasemismatch estimate.
 17. The method of claim 16, comprising providing saidcomplex training sequence on a closed RF signal path between atransmitter and a mixer of said receiver.
 18. The method of claim 16,comprising estimating said gain mismatch and said phase mismatchiteratively.
 19. The method of claim 16, comprising estimating said atleast one correction parameter iteratively.
 20. The method of claim 18,comprising utilizing a pre-compensated signal as a measurement signalfor said iterative estimation.
 21. The method of claim 18, comprisingperforming: post-correction using a latest available correctionparameter; and estimating residual mismatch from post-correctionsignals.
 22. The method of claim 21, comprising performing mismatchcompensation using matrix multiplication, during said post-correction.